Remove QWidgetStar.

Remove QWidgetStar.

This allows us to remove the odd hacks to get the static metaobject
for the QWidget* metatype.

The QWidget* is still an automatic metatype thanks to the QObject
partial template specialization. It is registered as a metatype
at runtime automatically in qwidgetsvariant.cpp.

Change-Id: Ie01b69eadf2cbe87af1a86c3284550f60dcf9e94
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>